Memory is considered one of the most important cognitive functions of humans.

From Cicero (first century BC) through Ebbinghaus (late nineteenth century) to the present, students of all ages have spoken of the memory, some with more success than others (taking into account the knowledge we have today) .

According to the Dictionary of the Spanish Language, memory is the "mental faculty through which retains and recalls the past."

Human memory is brain function resulting from synaptic connections between neurons through which humans can retain past experiences.

Memories are created when integrated into a circuit neurons reinforce the intensity of the synapse. "

Not only this is not proven, but involves a leap from the abstract to the material, ie not giving meaning as meaning but as an explanation of meaning.

Thus, the memory is to be retention and recall of facts to neuronal function that occurs when certain activities occur intra-and extraneuronal.

Markowitsch (2000) defined it as "a cognitive function that allows humans to record various information to the brain, stored and used when necessary or when required.

Memory allows the chaining and a temporary connection with other events, helping to unite experiences along the temporal evolution of the days, months and years. "

In any case, clearly define what memory is perhaps not as easy as at first appears, while other cognitive functions involved in the definition and execution.

Eichenbaum (2003) addresses the early research in memory of what he calls "the four C's", namely:

• Connection.

• Cognition.

• Compartmentalization.

• Consolidation.

In the first one-connection-Eichembaum draws attention to the study of memory from the circuitry that make it possible, by connecting the memory to give.

In other words, paying attention primarily to information processing systems, being at this level researchers concerned with how these connections occur and what the real needs of memory connection to occur.

Even as Eichembaun would try to "discover specific molecular events."

In cognition, psychologists, researchers, mainly in the field-leading the discussion and the field studies of associations, ie the processes by which the learning occurs, the mechanisms of stimulus-response, stimulus-reinforcement and other .

No matter whom this level of analysis supports the memory, but psychological (mainly cognitive) are the most concerned about whether the complexity of memory Eichenbaum say "may or may not be summarized in a few simple associative principles."

On the compartmentalization speaks of "memory location" and a fact that still some dare not say there is no single memory (as no single perception or a single praxia).

That is, the memory is segregated just as are the rest of cognitive functions. In the case of perception, they spoke of the segregation of gnosis (a patient may have color agnosia and other normal gnosis).

Similarly, memory is a function in which increasingly speaks of segregation (preservation of emotional memories and loss of memory of events, loss of memory of events that are related to time and that its recovery is necessary " time travel "as would Tulving, Memory and conservation facts that should not make that" journey ").

As for consolidation, refers to "when and how memories become permanent," meaning how facts become memory and make it permanently.

Is the question that haunts us: how is it possible that some cells do not save something material?

Much has been done in this regard and to describe the waterfalls cellular, molecular, and so on.,

Brain, yet this is a too vague and probably hundreds of data we are going to be determined from the descent into a world nuclear research or even subatomic.

Without more, then we review the main types of memory and then try the memory processes and the various diseases that affect memory processes, either directly or indirectly.

Main types of memory.

The early mnemonic models as we know them we owe to William James (1890), who made the first distinction between primary memory and secondary memory, which today are known from cognitive psychology as short-term memory and memory long term, respectively.

In sensory memory, information stays long enough to be attended to selectively and identified later processing.

The memory stores data and records that lasts a short time (between 200 and 300 msec), and data can be either visual (iconic), auditory (echoic), olfactory ... ..

This type of memory has been much discussed by various authors, since there is no clear evidence (if evidence, but contradictory) of his "existence" as an independent entity.

In fact, in the classic book by Eichenbaum (2.003) on memory, there is no reference to sensory memory.

Short-term memory

Once you have selected and attended a sensory memory information, go to a store of memory that is known with different nomenclatures: short-term memory, working memory, immediate memory, working memory or primary memory.

Baddeley and Hitch (1974) expanded the concept of primary memory or short-term memory, introducing the concept of working memory or working memory (working memory).

With this new terminology, refers to a system that performs two functions. On the one hand, it is able to maintain information in mind that information not being present.

On the other hand, you can manipulate that information allowing others involved in higher cognitive processes.

This concept does not have all the clarity it needs a concept in neuropsychology.

The Baddeley and Hitch model has undergone a reformulation (Baddeley, 2000), and has been working memory fragmented into four distinct subcomponents:

Phonological loop "refers to a process of review based on the control joint.

This subcomponent acts therefore as a temporary storage system that lets you use subvocal system until your brain to process that information.

It is important for the temporary storage of verbal material and to maintain the inner speech that is involved in short-term memory "(Muñoz-Céspedes Tirapu and 2005).

The phonological loop intervene in reading or in learning a phone number.

Visuospatial Agenda "operates similar to the phonological loop, only that its content is focused on maintaining and manipulating visual images" (Muñoz-Céspedes and Tirapu, 2005).

The agenda intervene in visuospatial imaging, or learning a route.

Episodic Buffer "phonological and visual information is combined in some way, and also integrates information coming from the long-term memory.

It is ultimately a system where information is stored simultaneously in the first two components and the long-term memory, so that creates a temporary multimodal representation of the current situation "(Muñoz-Céspedes and Tirapu, 2005 ).

Executive system (analogous to the Supervisory Attentional System model-SAS-of Shallice and Norman) "is a system by which to perform cognitive tasks involved in working memory, and that performs control and selection strategies "(Muñoz-Céspedes and Tirapu, 2005).

The short-term memory can not be reduced to a passive storage system in the short term, working memory serves and works as a system of limited capacity (7 + -2 items according to some authors), capable of storing but also to manipulate information, allowing the performance of cognitive tasks such as reasoning, comprehension and problem solving, thanks to the maintenance and seasonal availability of information (Roger Gil, 1.999).

Explicit or declarative memory includes an objective knowledge of the people, places and things and what it means.

This is reminiscent of a conscious and deliberate.

It is very flexible and affects the association of multiple bits and pieces of information (Kandel, 2.001).

The acquisition is related to the hippocampal system and other medial temporal lobe structures of the brain.

The hippocampus is only a temporary station on the path to long-term memory.

The long-term storage of episodic and semantic knowledge lies in different areas of the cerebral cortex (Morgado, 2005).

Therefore, the association areas are the last reservoirs of explicit memory, so that damage to the association cortex destroy or alter the memory of explicit knowledge that was acquired before the injury (Kandel, 2.001).

Tulving was the first to classify explicit episodic memory and semantic.

Episodic memory

Episodic memory allows humans to store and recall events that occurred during our lives, whether in our personal, family or social sphere.

This episodic memory is subject to a temporal-spatial reference (eg. What dined yesterday).

When we say that is subject to a temporal-reference refers to that memory is subject to a time (yesterday) and space (at home).

The areas of the neocortex that appear to be specialized in long-term storage of episodic knowledge are the association areas of the frontal lobes.

These prefrontal areas are working with other areas of the neocortex to allow the memory of where and when an event occurred (Kandel, 2.001).

Semantic memory

Semantic memory refers to knowledge, "knowledge", "culture" of a person, information acquired through books and education from the school.

Not subject to a reference spacetime (What is the capital of France?), Can not remember where, and what day we acquire that information.

Studies of patients with damage to the association cortex have shown that different representations of an object are stored separately. Semantic knowledge is not stored in a single region.

Rather, each time the knowledge about something is remembered, the memory is constructed from different pieces of information, each of which is stored in specialized memory pools.

As a result, damage to specific cortical area can lead to loss of specific information and, therefore, fragmentation of knowledge (Kandel, 2.001).

Memory processes.

Memory can be defined as a set of brain structures and cognitive processes that allow to set (encoding), save (storage) and recall (retrieval) very different crowd and information, and makes it possible to recognize as family events, recall past events or keep information long enough to be used immediately.

If any of these stages an error occurred or failure would lead to oblivion.

Fixation or encoding

The coding or fixation is the process where the information is prepared in order to be saved.

It is very important at this stage, concentration, attention and motivation of the subject.

At this early stage, the information can be encoded in multiple ways (an image, sounds, experiences, events or ideas), depending on the type of information and the strategies used by the subject.

Tulving and Thomson (1973) formulated the encoding specificity principle, which states that there is a close relationship between the coding of items in memory and its recovery.

In this sense, any key associated with an element during the coding phase, facilitate recovery in the subsequent recall phase (Ballestero, 1999).

Storage or consolidation

When the information is encoded in the previous stage must be stored in memory, so that information can be preserved in memory for a long time.

At this stage are important strategies used by the subject to store information (sorting, categorizing information, etc..) While carrying out the process.

Recovery or evocation

It refers to the processes that allow us to locate, access and use information that has been stored previously.

It is the process by which we recover the information.

If the information in the previous steps properly stored, when we want access to this information, it will be easier to find and use when required.

Main amnesic syndromes.

The first person who obtained evidence that memory processes could be located in specific regions of the human brain was the neurosurgeon Wilder Penfield.

But did not convince the scientific community of the time (Kandel, 2001, g).

As discussed above, there are different types of memory and other brain areas involved.

From studies of amnesic patients, we have to know the possible dissociation between different types of memory.

Classic amnesic syndrome

A classic amnestic syndrome may be due to an injury or involvement of the medial temporal lobe (encephalitis herpetic, Alzheimer's disease, transient global amnesia, hipocampectomías bitemporal and ischemic stroke) and also by diencephalic involvement (Korsakoff syndrome)

Temporal amnesic syndrome

In the medial temporal lobe amnesia, the most studied is the famous patient HM. Presented an anterograde episodic and semantic amnesia and retrograde amnesia. On the contrary, remained preserved short-term memory and procedural memory or implied.

Diencephalic amnesic syndrome

In the diencephalic amnesia, the most studied cases are patients with Korsakoff syndrome.

They have both anterograde amnesia semantics as episodic, but not always explained by an inability of storage, but may relate in part to a memory disorder, which is an element of distinction between the medial temporal lobe amnesic and diencephalic .

Also have a retrograde amnesia (Roger Gil, 1999).

Plots are typical, they act as if they remembered situations that really do not remember or invent facts that have never happened in reality.

They have difficulty making an assessment of their own memory capacity (metamemory). They have, on memory tests, interference and intrusion.

Front amnesic syndrome

The frontal cortex lesions do not produce a general loss of memory.

However, these patients may present some problems when they bring into play the skills involved in search and selection of information relevant to each situation.

In short, the frontal cortex plays a fundamental role in organizing, searching, selection and verification of memories of the stored information.

Therefore not involved in storage processes, but average recovery strategic processes, monitoring and verification (Tirapu and Muñoz-Céspedes, 2005).

One possible interpretation of this phenomenon is that the free recall test internal search strategies require information that these individuals can not produce themselves (Perea, 2,001).

Focal retrograde amnesia

Patients with this type of amnesia, have a significant effect of retrograde declarative memory, as well as an impairment of declarative anterograde memory may vary in severity.

The other types of memory and cognitive functions other are preserved.

This type of amnesia is associated with lesions of temporal areas that do not affect the hippocampus. The most common causes are head injury, temporal lobe epilepsy, psychogenic amnesia and vertebrobasilar insufficiency.

In order to reach a differential diagnosis between the various amnesic syndrome, we must perform a neuropsychological evaluation to analyze qualitatively different memory systems, to know what processes are impaired and preserved, and reach out a program of rehabilitation / most appropriate stimulation, if applicable.

One of the most comprehensive tests to study this type of memory is that of Buschke Selective Reminding. This test explores the three fundamental processes of memory (setting, consolidation and recall), through free recall and facilitated.

The subject is presented with a series of words or pictures, or names that reads aloud, then is given a semantic key for each item.

During the facilitated memory provides the semantic key words that has not recalled in free recall.

That key is used to access the material, if it has been stored properly, and recall that information.

For a detailed presentation of the tests are recommended manual Lezak (1995).